This invention relates to methods and equipment for treatment and remediation of contaminated soil and groundwater, with emphasis on improved in-situ methods and equipment for efficiently delivering of active treatment agents to the contaminated region.
A primary objective of environmental remediation of subsurface contamination is to remove or treat the contamination when possible; and when removal or treatment is not possible, to provide interim containment of mobile hazardous and/or radioactive waste constituents. Examples of contaminants of concern include pesticide-contaminated soil and/or groundwater, benzene vapors, or non-aqueous phase liquids, such as gasoline leaking from a buried storage tank. An underground barrier, whether permeable or impermeable, can be used to contain, remediate, and/or redirect the flow of contaminated groundwater. An impermeable subsurface barrier wall is typically made of a substantially impermeable material that prevents the migration of mobile waste forms through the relatively permeable surrounding ground (soil, sand, etc.). Neat cement-based grout (a well-known mixture of Portland cement and water), mixed with the surrounding soil, is commonly used as a ground-hardening material to fabricate impermeable underground barrier walls. The neat cement is vigorously mixed with the soil to ensure the soil aggregate acts as a binder. The cement/soil mixture can be used to backfill trenches that have been dug (which is limited to a depth of less than 40 feet).
Advancing this technology, the underground barrier wall can be constructed of reactive materials that actively remediate (i.e., remove or transform) contaminants, as opposed to simply blocking movement or confining with impermeable barriers. Conceptually, permeable reactive barriers (PRB""s) are used to intercept and remove ground water contaminants in-situ before passing into the wider ecosystem (see FIG. 1). The barrier can be constructed of benign permeable reactive media (PRM), such as small, solid particles of zero-valent (metallic) iron that are used to breakdown or immobilize contaminants (e.g., by redox reduction) using ordinary chemical, physical, and/or biological means (the word xe2x80x9cbarrierxe2x80x9d is defined herein to include zones that are permeable to the passage of water, other liquids, or gases). Treated, uncontaminated groundwater then exits from the barrier and is returned to the aquifer.
Currently, reactive barrier technology is practically limited to use in excavated trenches at depths of less than 40 feet, due to problems with injecting dry particulate media to greater depths using drill strings. Experimental attempts at greater depth placement have used fluid-based viscosifying agents to inject PRM into subsurface regions. Viscosifying/suspension agents (e.g., organic guar, with a viscosity like molasses) have been used to suspend the often-dense PRM (e.g., iron particles) to facilitate pumping and injection using high-pressure systems. Problems with this method include non-uniformity of the mixture and retention of the viscosifier in the treatment zone, potentially causing permeability reduction problems, biological growth abnormalities, and other problems.
Underground reactive barrier walls (or zones) can be fabricated in-situ using modified (i.e., hybrid) jet grouting techniques. The term xe2x80x9cjet groutingxe2x80x9d refers to the use of one or more high-pressure jet spray nozzles, which are located on a jet grout injector sub-assembly attached to the end of a drill string, to inject slurry-like materials at relatively high velocity radially outwards into the surrounding soil. The high-velocity jet spray simultaneously masticates and erodes the surrounding soil, while dispersing and blending the injected slurry with the loosened soil. If the slurry is primarily made of cement-based grout, then the mixture of soil and grout subsequently hardens into a solid, substantially impermeable material, sometimes called xe2x80x9csoilcretexe2x80x9d. In the case of the advanced technology the jetted reagent material contains PRM""s, and the mixture of soil and PRM (or, the replacement of loosened soil with PRM""s) forms a permeable reactive barrier zone.
Traditionally, the jet grout injector is rotated during withdrawal (by rotating the drill string), creating a cylindrical column (typically in the vertical direction). On the other hand, if the jet grout drill string is not rotated during withdrawal, then the jet spray creates a relatively thin panel/wall section. Multiple columns or wall panels can be emplaced underground in an overlapping pattern to create a continuous barrier wall or zone.
Conventionally, three different generic types of jet grouting injector sub-assemblies are currently used for injecting neat cement grout i.e., one-fluid, two-fluid, and three-fluid designs. A one-fluid design has a single outlet nozzle that radially sprays a slurry mixture containing grout. A two-fluid design has two coaxial outlet nozzles, where the inner nozzle sprays the grout slurry and the outer nozzle sprays a conical aureole of compressed air, which serves to increase the radial distance of jet cutting action. A three-fluid design has three nozzles. Two of these nozzles are coaxial; with the inner nozzle injecting high-velocity water, and with the outer coaxial nozzle injecting a conical aureole of compressed air. The combined action of the water jet surrounded by the conical aureole of compressed air cuts and masticates the surrounding soil. The third nozzle, located some distance below the upper two coaxial nozzles, injects a relatively low-pressure slurry of grout into the already-masticated, loosened soil. In the two-fluid and three-fluid designs, the use of compressed air increases the radius of influence of the water jet. It also xe2x80x9clightensxe2x80x9d the mixture of soil and water in the zone of influence of the jet, thus creating an airlift which pumps excess water and soil fines through the annular space between the borehole wall and the drill string to the surface. However, the single-fluid design is generally preferred for drilling inclined and horizontal holes.
For construction of permeable reactive barrier zones to depths greater than 40 feet, drilling operations use drill bits such as tri-cone, drag, and rock bits mounted on the end of a jet grouting nozzle sub-assembly (xe2x80x9csubxe2x80x9d or xe2x80x9cmonitorxe2x80x9d). Drilling bits use air or water or bentonite mud as a bit lubricant/cooling medium, which also serves as a carrier medium to bring cuttings up out of the hole during the drilling operation (i.e., spoils). Down-the-hole (DTH) hammer drill assemblies can also be used, which are driven by compressed air to provide a repetitive, percussive hammering force on the drill bit to increase drilling rates. The compressed air exits the DTHH drill assembly through passages in the attached drill bit, which expels rock cuttings from around the drill bit and flushes them away.
Downhole injection of PRM""s mixed with liquid-based suspension agents results in multiple problems and concerns, which center around the practice of using jet grouting nozzles that were designed only for injecting neat cement/grout slurries.
Melegari, in U.S. Pat. No. 5,624,209, teaches that permeable reactive media made of dry aggregate particles can be suspended in a stream of air under relatively low pressure (approx. 20 bar), and then co-injected simultaneously with a separate spray of high-pressure water (approx. 500 bar) using a pair of coaxial radial nozzles, where the high-pressure water is injected through the inner nozzle, and the air/media spray being injected through the outer nozzle. The high-pressure/high-velocity (approx. 200-250 m/s) water jet disintegrates the soil and produces a mixing effect that disperses the injected dry media in the soil. The use of only low-pressure air to suspend the solid particles limits the ability of the device to inject the particulate matter deeply into the soil (in a radial direction).
Melagari explains in related U.S. Pat. No. 5,944,454 that the size of solid particles suspended in his air stream is preferably a few microns, and not more than 10 microns. For larger particles, such as steel granules having a size of several millimeters, Melagari teaches that it is not possible to obtain a stable and evenly distributed mixture of these millimeter-size solid substances in air at high pressure, nor is it possible to mix these solid particles with a high-pressure fluid (e.g., water) when the injection pressure is higher than 70-80 bar, due to the phenomena of xe2x80x9cpresso-filtrationxe2x80x9d that lead to the rising of compact layers inside the pipes that close the passage.
We have performed experiments using a coaxial nozzle geometry similar to Melegari""s. Our tests revealed severe problems due to severe mechanical erosion of the internal passageways conveying the mixture of air+solid particles (i.e., the air/media stream). Excessive erosion was observed on both the inner and outer coaxial nozzles due to the abrasive action of the impinging solid particles, which ultimately lead to failure of the system. Also, clogging of narrow passages can occur if wet air (i.e., non-dry air) is used to suspend the particles.
Alternatively, Melagari teaches in U.S. Pat. No. 5,944,454 an apparatus for suspending the solid particles (e.g., having diameters 4-5 mm or more) in high density/high viscosity xe2x80x9cgelatinizedxe2x80x9d water (water plus gelling agent, such as a high molecular weight organic polymer) and for co-injecting the suspended particles at low-pressure (50-100 bar) through the outer nozzle of a coaxial radial nozzle arrangement, simultaneously while injecting high-pressure water (300-600 bar) through the inner coaxial radial nozzle.
However, pumping of PRM""s suspended in a viscous, high-density liquid requires time-consuming field operations, such as lengthy mixing procedures, to adequately combine and suspend the PRM in a liquid carrier (e.g., gelatinized water). Other problems with this method include: 1) pumping system internal wear caused by forcing abrasive PRM agents suspended in liquid through the pump internals; 2) continual hose and jet nozzle obstructions and clogging due to poorly mixed PRM with fluid suspension agents; 3) inability to accurately measure injected volume of PRM reagent; 4) inadvertent introduction of exogenous bacterial food supply with subsequent bacteriologic contamination of groundwater from organic suspension agents (e.g., guar gum, gelatinized water, etc.); 5) inability to accurately measure total liquid volumes injected, since pumps, hoses, pipes, and drill pipe retain large unaccounted volumes; and 6) excessive spoils return to the surface via the drill string hole annulus (sometimes as high as 70% spoils returns), which results in unwanted transport of contaminated soil to the surface, and in recycling PRM back to the surface, rather than treating the contaminants of concern in-situ.
Alternatively, Melegari teaches in U.S. Pat. No. 6,050,337 a jet grout injector sub-assembly that has three or more radial nozzles stacked axially (i.e., vertically) along the drill string. The first nozzle (located at the highest level on the sub-assembly) injects a high-pressure fluid (e.g., water at 600 bars and more) to break the soil with a high-velocity jet (e.g., 300-350 m/s). The second nozzle (the one in the middle) injects particulate material at a low pressure. The third nozzle (the one on the bottom) injects high-pressure water to mix the injected particulate media with the ground broken up by the first nozzle.
Melegari continues to teach in U.S. Pat. No. 6,050,337 a treatment method where the high-pressure fluid is first injected by the top nozzle; then the particulate material is injected at low pressure through the middle nozzle; followed sequentially by injecting the high pressure fluid to mix the injected material with the broken soil through the bottom nozzle.
However, the distances between adjacent nozzles in Melegari""s triple nozzle design are so large that essentially no interaction can occur between adjacent jets/sprays. In other words, the action of each jet/spray at a given depth in the soil occurs independently of the other one (i.e., each jet/spray passes by the given location/height sequentially one-at-a-time as the drill string is withdrawn). One problem with the large spacing between nozzles taught by Melegari is that masticated soil broken by the uppermost high-pressure water nozzle can interfere with, and limit, the dispersing power of the middle nozzle. Additionally, the bottom (water spray) nozzle is spaced so far down below the middle (media injection) nozzle that no synergistic effect can occur between the middle and bottom nozzles that would lead to a greater radial distance of mixing (as is well-known in conventional two-fluid jet grouting systems that use a conical aureole of compressed air coaxially surrounding an inner high-pressure jet spray of grout to increase the radial distance of grout deposition by roughly a factor of two as compared to a single-fluid system that doesn""t use an aureole of compressed air).
For some drilling applications, a Down-the-Hole (DTH) hammer drill assembly is used, for example, to increase drilling rates. The DTHH drill assembly uses compressed air to provide a repetitive, percussive hammering force onto the drill bit. This requires an independent supply of compressed air. The use of a coaxial pipe string, such as taught by Melegari, supra, cannot provide a third independent supply of compressed air, while simultaneously supplying the two different fluids described above (i.e., for cutting/mixing with a water jet, and for injecting solid particles suspended in air or gelatinized water).
A need remains, therefore, for a simple, readily deployable, and long-lasting solution to the problem of efficiently injecting permeable reactive media (PRM) to form permeable reactive barriers (PRB""s) having a large radial depth of penetration, a uniform and homogenous distribution of PRM, an engineered permeability and hydraulic conductivity, etc., while minimizing problems with excessive return of spoils to the surface, and while eliminating problems due to bacterial contamination and clogging of the treated subsurface volume caused by the use of organic liquid-based suspension agents. A need also exists for a media injector sub-assembly that can accommodate three independent fluid/gas streams (e.g., compressed air, high-pressure water, and particulate media suspended in a carrier gas or liquid) for use with a DTH hammer drilling assembly.
Against this background, the present invention was developed.
The present invention relates to an improved method and apparatus for injecting particulate media into the ground for constructing underground permeable reactive barriers, which are used for environmental remediation of subsurface contaminated soil and water. A media injector sub-assembly can be attached to a triple wall drill string pipe, which sprays a mixture of active particulate media suspended in a carrier fluid radially outwards from the sub-assembly, at the same time that a mixing fluid is sprayed radially outwards. The media spray intersects the mixing spray at a relatively close distance from the point of injection, which entrains the particulate media into the mixing spray and ensures a uniform and deep dispersion of the active media in the surrounding soil. The media injector sub-assembly can optionally include channels for supplying compressed air to an attached down-the-hole hammer drive assembly for use during drilling.